Directional solidification furnaces are used to produce multi-crystalline silicon ingots. During use of the furnace, raw silicon is first placed into a crucible disposed in the furnace. The raw silicon can take the form of solid chunks, recycled polysilicon, silicon dust, or a combination thereof. The crucible is typically constructed of quartz or another suitable material that can withstand high temperatures. The crucible is typically a five-sided box, with a lid placed over an open top of the box. The lid is typically not in direct contact with the box. The crucible is supported by a support structure that adds structural rigidity to the crucible.
The crucible is disposed within a containment vessel that forms part of the furnace. The containment vessel seals the crucible from the outside ambient environment. The pressure of the ambient atmosphere within the containment vessel is reduced after the crucible has been charged with silicon and sealed within the vessel. The content of the ambient atmosphere within the containment vessel can also be monitored and controlled.
The crucible and the charge are then heated to a temperature sufficient to melt the silicon. After the charge has completely melted, it is cooled at a controlled rate to achieve a directional solidification structure within the resulting ingot. The controlled rate of cooling is established by any combination of reducing the amount of heat applied by the heaters, movement of or the opening of a heat vent in insulation surrounding the crucible, or the circulation of a cooling medium through a cooling plate disposed adjacent the crucible. All of these methods results in the transfer of heat away from the crucible.
If the rate of cooling of the bottom of the crucible is greater than that of the sides of crucible (i.e., if the heat loss through the sides of the crucible is near zero), then a relatively flat, horizontal solidification isotherm is generated. The ingot thereby solidifies in the region closest to the cooler side of the crucible and proceeds in a direction away from the side of the crucible. The last portion of the melt to solidify is generally at the top of the ingot if a relatively flat, horizontal solidification isotherm was generated.
A concern in the production of multi-crystalline ingots is the rate of heat transfer from the melt during solidification. If a flat, horizontal solidification isotherm does not develop during solidification, different portions of the melt within the same plane can solidify at different times. This differential solidification of the ingot within the same plane can in turn lead to the formation of defects within the ingot.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.